My guess is that they will run into Tavia's group and find solitude there for a bit until Carver's group tracks them down and brings it to desolation. That is what I think will lead to 'Amid the Ruins'; makes sense of the title at least. This will probably all build up to a war between the two groups with various characters from 400 days on either side depending on your choices from the DLC. I think that Nate will be a a member of Carver's group, either as a follower of Carver or perhaps even as his leader, and was on a recruitment/supply mission when he met Russel. Tavia's camp definitely seems to be the connecting thread though between Clem's group and the 400 days characters.